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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

ZINC AND  ITS ALLOYS

241

TABLE  XXVII
Composition of Aluminium-bronzes

2  (for automobiles)

3                do.

4                do.

Cu

92-96
94'9
95-4
97-0

Al

Si

Fe

4-8
	I
	!

4-7
	0-41
	;         O-IO

4-0
	i       °'33
	O-I2

2-7
	|       0-19
	i          O-20

ZINC   AND  ITS   ALLOYS

In this place zinc as such will be treated and methods described for
determining its common impurities ; technical tests for zinc dust are also
given. As regards the numerous alloys in which zinc occurs, those with
copper are dealt with under copper and its alloys and those with nickel under
nickel and its alloys.

ZINC

The extraneous elements commonly found in commercial zinc are lead;
iron and cadmium. Small quantities of arsenic, antimony and sulphur
are also often found and in remelted zinc also tin, but only rarely are traces
of carbon, silicon, phosphorus, copper, nickel, cobalt, etc., found.

The essential determinations to be made on commercial zinc are those
of lead, iron and cadmium. It may sometimes be of interest to estimate
the arsenic, antimony and sulphur, but as a rule it is sufficient to test quali-
tatively for the other elements.

1. Determination of the Lead, Iron and Cadmium (according to
Mylius and Fromm x).—To 100 grams of the sample and 200 c.c. of water,
in a flask holding about 2 litres, nitric acid is added gradually until, with
gentle heating, the metal is completely attacked.

The solution obtained is then treated with ammonia until the zinc
hydroxide at first separating redissolves, and then diluted to about two litres,
very dilute ammonium sulphide being next added in small quantities and
with stirring until the lead, iron and cadmium are completely precipitated
and zinc sulphide begins to appear.2 The liquid is kept at 80° for a short
time to facilitate separation of the precipitate, which consists of lead, cad-
mium, iron, etc., sulphides mixed with zinc sulphide, and is filtered off when
the supernatant solution becomes clear. The precipitate is treated on the
filter with hot, dilute hydrochloric acid,3 which dissolves lead, cadmium

1  Zeitschr. analyt. Chem., 1897, p. 37.

2  A small portion of the liquid, filtered and treated with ammonium sulphide, should
yield a distinctly white precipitate.

3  aN-Hydrochloric acid (20 c.c. of cone. HC1 made up to too c.c.).    It is advisable
to place the acid in a wash-bottle and to wash with it also the flask in which the precipi-
tation took place.    About 150-200 c.c.* to be used in all.

A.C.                                                        "                                                          16issolved in aqua regia or in hydrochloric